3 Answers

Good q .well ,u need dark ,ghost and bug type moves to defeat him because he has psychic type pokes and these are their weaknesses .a tm like shadow ball ,dark pulse ,x-scissors or shadow claw would be useful .as mew said before me a fire type move would be useful to beat his bronzong .i hope that you will defeat him .Cynthia is even harder to defeat .

Use bug, ghost and dark types, which are effective to psychic Pokemon. Bronzong is only weak to fire attacks. After you defeat the elite four, you met cynthia, she use several types of Pokemon. She has got Spiritomb, Gastrodon, Roserade, Milotic, Lucario and Garchomp.
Hope this helps!